The present invention relates to an exhaust gas recirculation system for an internal combustion engine for a vehicle such as an automobile, and, more particularly, to an exhaust gas recirculation system of the back pressure control type.
An exhaust gas recirculation system of the back pressure type, which is known as a system of exhaust gas recirculation for an internal combustion engine, generally comprises an exhaust gas recirculation control valve provided at a middle portion of a recirculation passage which recirculates a part of exhaust gases from the exhaust passage to the intake passage of an engine, said control valve having a diaphragm chamber and being adapted to increase its opening in accordance with increase of vacuum supplied to said diaphragm chamber, a vacuum port provided in the intake passage so as to be located upstream of a throttle valve incorporated in the intake passage when it is fully closed and so as to be located downstream of said throttle valve when it is opened beyond a predetermined opening, and a vacuum control valve provided at a middle portion of a vacuum passage which connects said vacuum port and said diaphragm chamber of said exhaust gas recirculation control valve and being adapted to control the vacuum conducted through said vacuum passage in accordance with the pressure of exhaust gases in the recirculation passage, wherein the exhaust gas recirculation control valve is operated by the vacuum which is supplied from said vacuum port and controlled by said vacuum control valve in accordance with the pressure of exhaust gases in the recirculation passage so that the exhaust gas recirculation ratio, i.e. the ratio of flow of exhaust gases to that of intake air, is maintained substantially at a constant value over a wide range of operation of the engine.
However, in the conventional exhaust gas recirculation system of the back pressure type having the abovementioned structure, since the exhaust gas recirculation control valve is operated by the vacuum controlled in accordance with the pressure of exhaust gases in the recirculation passage, there occurs a time delay from the instant when the flow of intake air is changed to the instant when the exhaust gas recirculation control valve is operated to meet with the change of intake air flow due to the time required for engine intake air to flow through the cylinders of the engine. Furthermore, a delay in the response of the exhaust gas recirculation control valve is also caused by the throttling means provided in the vacuum passage for conducting vacuum from the vacuum port to the diaphragm chamber of the exhaust gas recirculation control valve and the throttling means provided in the recirculation passage for conducting exhaust gases from an exhaust gas takeout port provided in the exhaust passage to a back pressure chamber located upstream of the exhaust gas recirculation control valve. Therefore, in transient operational conditions such as acceleration or deceleration, changes of flow of exhaust gases are effected with a substantial delay relative to changes of flow of intake air. Such a delay in response causes no serious problem in acceleration, whereas in deceleration, particularly in abrupt deceleration, it causes a serious problem in that when the flow of intake air has already been decreased, a high exhaust gas pressure still exists in the back pressure chamber, whereby the vacuum control valve is maintained in such an operating condition as to hold a high vacuum in the vacuum passage for the diaphragm chamber of the exhaust gas recirculation control valve, while such a high vacuum in the vacuum passage is not immediately relieved through the vacuum port now located upstream of the fully closed throttle valve due to the throttling means provided at a middle portion of the vacuum passage. Therefore, in abrupt deceleration, a large amount of exhaust gases is recirculated to a very small amount of intake air, thereby causing misfiring of the internal combustion engine. Furthermore, when the exhaust gas recirculation control valve has been throttled to meet with the reduction of intake air flow after a time of delay in response, the engine is then abruptly restored to its normal operation, thereby causing forward jerking of the vehicle, which makes it less controllable. These problems become more serious, the higher is the exhaust gas recirculation ratio.